Coordination Engineering of Dual Co, Ni Active Sites in N-Doped Carbon Fostering Reversible Oxygen Electrocatalysis

The development of affordable and non-noble-metal-based reversible oxygen electrocatalysts is required for renewable energy conversion and storage systems like metal-air batteries (MABs). However, the nonbifunctionality of most of the catalysts impedes their use in rechargeable MAB applications. Moreover, the loss of active sites also affects the long-term performance of the electrocatalyst toward oxygen electrocatalysis. In this work, we report a simplistic yet controllable chemical approach for the synthesis of dual transitional metals such as cobalt, nickel, and nitrogen-doped carbon (CoNi-NC) as bifunctional electrode materials for rechargeable zinc-air batteries (ZABs). The spatially isolated Ni-N4 and Co-N4 active units were rendered for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), respectively. The individual efficacy of both reversible reactions enables an ΔE value of ∼0.72 V, which outperforms several bifunctional electrocatalysts reported in the literature. The half-wave potential (E1/2) and overpotential were achieved at 0.83 V and 330 mV (vs RHE) for ORR and OER, respectively. The peak power density of ZAB equipped with the CoNi-NC catalyst was calculated to be 194 mW cm-2. The present strategy for the synthesis of bifunctional electrocatalysts with dual active sites offers prospects for developing electrochemical energy storage and conversion systems.

3D Co-Doping α-Ni(OH)2 Nanosheets for Ultrastable, High-Rate Ni-Zn Battery

The α-Ni(OH)₂ is regarded as one promising cathode for aqueous nickel-zinc batteries due to its high theoretical capacity of ≈480 mAh g^-1 , its practical deployment however suffers from the poor stability in strong alkaline solution, intrinsic low electrical conductivity as well as the retarded ionic diffusion. Herein, a 3D (three dimensional) macroporous α-Ni(OH)₂ nanosheets with Co doping is designed through a facile and easily scalable electroless plating combined with electrodeposition strategy. The unique micrometer-sized 3D pores come from Ni substrate and rich voids between Co-doping α-Ni(OH)₂ nanosheets can synergistically afford facile, interconnected ionic diffusion channels, sufficient free space for accommodating its volume changes during cycling; meanwhile, the Co-doping can stabilize the structural robustness of the α-Ni(OH)₂ in the alkaline electrolyte during cycling. Thus, the 3D α-Ni(OH)₂ shows a high capacity of 284 mAh g^-1 at 0.5 mA cm^-2 with an excellent retention of 78% even at 15 mA cm^-2 , and more than 2000 stable cycles at 6 mA cm^-2 , as well as the robust cycling upon various flexible batteries. This work provides a simple and efficient pathway to enhance the electrochemical performance of Ni-Zn batteries through improving ionic transport kinetics and stabilizing crystal structure of cathodes.

The Rate Performance of Two-Dimensional Material-Based Battery Electrodes May Not Be as Good as Commonly Believed

Two-dimensional (2D) materials show great potential for use in battery electrodes and are believed to be particularly promising for high-rate applications. However, there does not seem to be much hard evidence for the superior rate performance of 2D materials compared to non-2D materials. To examine this point, we have analyzed published rate-performance data for a wide range of 2D materials as well as non-2D materials for comparison. For each capacity-rate curve, we extract parameters that quantify performance which can then be analyzed using a simple mechanistic model. Contrary to expectations, by comparing a previously proposed figure of merit, we find 2D-based electrodes to be on average ∼40 times poorer in terms of rate performance than non-2D materials. This is not due to differences in solid-state diffusion times which were similarly distributed for 2D and non-2D materials. In fact, we found the main difference between 2D and non-2D materials is that ion mobility within the electrolyte-filled pores of the electrodes is significantly lower for 2D materials, a situation which we attribute to their high aspect ratios.

Two-Dimensional Materials to Address the Lithium Battery Challenges

Despite the ever-growing demand in safe and high power/energy density of Li⁺ ion and Li metal rechargeable batteries (LIBs), materials-related challenges are responsible for the majority of performance degradation in such batteries. These challenges include electrochemically induced phase transformations, repeated volume expansion and stress concentrations at interfaces, poor electrical and mechanical properties, low ionic conductivity, dendritic growth of Li, oxygen release and transition metal dissolution of cathodes, polysulfide shuttling in Li-sulfur batteries, and poor reversibility of lithium peroxide/superoxide products in Li-O₂ batteries. Owing to compelling physicochemical and structural properties, in recent years two-dimensional (2D) materials have emerged as promising candidates to address the challenges in LIBs. This Review highlights the cutting-edge advances of LIBs by using 2D materials as cathodes, anodes, separators, catalysts, current collectors, and electrolytes. It is shown that 2D materials can protect the electrode materials from pulverization, improve the synergy of Li⁺ ion deposition, facilitate Li⁺ ion flux through electrolyte and electrode/electrolyte interfaces, enhance thermal stability, block the lithium polysulfide species, and facilitate the formation/decomposition of Li-O₂ discharge products. This work facilitates the design of safe Li batteries with high energy and power density by using 2D materials.

Molecular structure and interactions of water intercalated in nickel hydroxide

The structure and properties of α-Ni(OH)2 containing water and nitrate have been investigated computationally. The adsorption of water molecules on the Ni(OH)2 surface is also investigated to provide insight into the nature of the water-Ni(OH)2 interactions. The spectroscopic and dynamical behaviour of the intercalated species has been characterized and used to explain experimental findings reported for this material. The results presented here indicate that the water molecules interact non-covalently with Ni(OH)2, with a binding energy that is comparable in magnitude with that of the water dimer hydrogen bond. The presence of the intercalated species increases the distance between the Ni(OH)2 layers such that the interlayer interactions are negligible. The weakening of the interlayer interactions facilitates the horizontal displacement of the layers relative to one another, providing a possible origin for stacking faults observed in α-Ni(OH)2. Comparison of the vibrational frequencies calculated here with the experimental spectra confirms that α-Ni(OH)2 containing only water molecules can be synthesized. The structures of the water molecules intercalated in α-Ni(OH)2 were found to be analogous to those absorbed in γ-NiOOH, while the water-layer interactions are stronger in γ-NiOOH. The results presented here characterize the structure and interactions of water intercalated in nickel hydroxides and also provide insights into the effects of intercalated water on the properties of layered metal hydroxides.

Two-Dimensional N,S-Codoped Carbon/Co9S8 Catalysts Derived from Co(OH)2 Nanosheets for Oxygen Reduction Reaction

The development of highly active and cost-efficient electrocatalysts for the oxygen reduction reaction (ORR) is of great importance in a wide range of clean energy devices, including fuel cells and metal-air batteries. Herein, the simultaneous formation of Co₉S₈ and N,S-codoped carbon with high ORR catalytic activity was achieved in an efficient strategy with a dual templates system. First, Co(OH)₂ nanosheets and tetraethyl orthosilicate were utilized to direct the formation of two-dimensional carbon precursors, which were then dispersed into thiourea solution. After subsequent pyrolysis and template removal, N,S-codoped porous carbon-sheet-confined Co₉S₈ catalysts (Co₉S₈/NSC) were obtained. Owing to the morphological and compositional advantages as well as the synergistic effects, the resultant Co₉S₈/NSC catalysts with a modified doping level and pyrolysis degree exhibit superior ORR catalytic activity and long-term stability compared with the state-of-the-art Pt/C catalysts in alkaline media. Remarkably, the as-prepared carbon composites also reveal exceptional tolerance of methanol, indicating their potential applications in fuel cells.

One material, multiple functions: graphene/Ni(OH)2 thin films applied in batteries, electrochromism and sensors

Different nanocomposites between reduced graphene oxide (rGO) and Ni(OH)₂ nanoparticles were synthesized through modifications in the polyol method (starting from graphene oxide (GO) dispersion in ethylene glycol and nickel acetate), processed as thin films through the liquid-liquid interfacial route, homogeneously deposited over transparent electrodes and spectroscopically, microscopically and electrochemically characterized. The thin and transparent nanocomposite films (112 to 513 nm thickness, 62.6 to 19.9% transmittance at 550 nm) consist of α-Ni(OH)₂ nanoparticles (mean diameter of 4.9 nm) homogeneously decorating the rGO sheets. As a control sample, neat Ni(OH)₂ was prepared in the same way, consisting of porous nanoparticles with diameter ranging from 30 to 80 nm. The nanocomposite thin films present multifunctionality and they were applied as electrodes to alkaline batteries, as electrochromic material and as active component to electrochemical sensor to glycerol. In all the cases the nanocomposite films presented better performances when compared to the neat Ni(OH)₂ nanoparticles, showing energy and power of 43.7 W h kg⁻¹ and 4.8 kW kg⁻¹ (8.24 A g⁻¹) respectively, electrochromic efficiency reaching 70 cm² C⁻¹ and limit of detection as low as 15.4 ± 1.2 μmol L⁻¹.

Self-Assembly of Parallelly Aligned NiO Hierarchical Nanostructures with Ultrathin Nanosheet Subunits for Electrochemical Supercapacitor Applications

Parallelly aligned NiO hierarchical nanostructures were fabricated using a templated self-assembly method followed by calcinations, where rationally employed pluronic triblock copolymers (P123) are acting as molecular templates for geometrical manipulation of nanocrystals and short-chain alcohols are acting as cosolvents and cosurfactants. Such aligned nanostructure is constructed orderly with several ultrathin two-dimensional (2D) nanosheet subunits with an exceptionally small thickness of only 3 nm in a high degree of orientation and separation. Moreover, the number of assembled nanosheets in a unit can be tuned by changing the concentration of the involving P123. This is the first time to synthesize highly hierarchically ordered and bilaterally symmetrical nanostructures, distributed in diameter of around 200-300 nm, via self-assembly in the liquid phase without solid substrates. The as-synthesized NiO delivered high capacitances of 418 F/g at the current density of 2 A/g with well cycling stability (still maintained 85% after 2000 cycles) and 333 F/g at 10 A/g in rates performance after 60 cycles. These fine electrochemical performances are supposed to be attributed to the hierarchical structures with high specific surface area (SSA, ∼164.87 m(2)/g) and ordered multilevel mesopores, which facilitate the electrolyte accessibility and provide more active sites for redox reaction.

Water-floating nanohybrid films of layered titanate–graphene for sanitization of algae without secondary pollution

A novel efficient and safe methodology to sanitize algae in natural water without secondary pollution is developed by fabricating floating graphene–inorganic hybrid films.